Horatio searches for the Mala Noche gang member who ordered Marisol's murder. In addition, five Mala Noches are gunned down at one of their safe houses. Back at the lab, FBI agents claim that someone stole $12,000 that was confiscated from a recent crime scene. As their probe intensifies, the department mole is revealed. Finally, Horatio learns a surprising fact about Marisol.
Season 4, Episode 24
A member of the Mala Noche gang escapes from the courtroom during his own trial and the gang order a hit on Horatio, but they instead shoot someone close to him. Meanwhile, Delko deals with a jilted former lover who seeks to hurt the other women in his life and who believes that Marisol is his wife, not his sister.
Season 4, Episode 23
A young socialite who is famous for being famous is murdered during a party at her lush oceanfront house, electrocuted in her own bathtub with a tanning lamp. The investigation reveals that the victim's rival was at the party and that the dead woman had a drug problem. The probe leads Horatio to uncover a drug ring that uses kidnapped women from El Salvador as slave labor. Also, Horatio gives Delko some surprising news about himself and Marisol.
Season 4, Episode 22
A newlywed who was married on a cruise ship dies after he is pushed overboard by someone and then attacked by sharks. The investigation reveals that the victim was a millionaire and that $70,000 is missing from his room's safe. It's also learned that he was drugged and that his bride of three days has a 16-year-old daughter. Back on dry land, Marisol is harassed by a federal agent who questions her about Horatio and the lab.
Season 4, Episode 21
Horatio searches for a kidnapped woman after the victim makes a random cell-phone call from inside the trunk of a car pleading for help. The investigation reveals that she's being held for ransom and that the kidnappers want $5 million or they will kill her. Back at the lab, Delko learns that Natalia threw a party but didn't invite him and that Ryan is interested in asking Natalia out.
Season 4, Episode 20
Two brothers, both shot in the back of the head, are found in a bathtub in a dilapidated hotel that is scheduled to be gutted and then remodeled. One of the victims is dead, but the other one clings to life. The investigation reveals the shootings are linked to a drug dealer's money-laundering scheme and to a previous case in which two young lovers stole a car and some cash outside of a bank. Also, Ryan worries a news reporter is getting too involved in the story for her own good.
Season 4, Episode 19
Three masked men rob women at a Coconut Grove day spa, one of whom is shot when she fights back with pepper spray. When Horatio arrives on the scene, he learns that another woman injured in the attack is Marisol. The investigation becomes more urgent when it is discovered that the shooter is now breaking into the women's homes. Later, Horatio and Marisol make a surprising decision.
A man goes on trial for his wife's murder, even though her body was never found, and the jury finds him to be not guilty. But shortly after the suspect goes free, the victim's body turns up in a fishing boat's net and Horatio becomes determined to put the man away. Horatio comes to believe that the suspect committed a so-called practice murder to prepare himself to kill his wife. The investigation also reveals that the wife's killing may have been racially motivated.
Season 4, Episode 17
A woman is killed when her car smashes into a barrier. When Horatio arrives on the scene, he discovers a man's dead body in the car's trunk. The investigation reveals that another vehicle was involved in the wreck and that a $4-million necklace was stolen from the crime scene. Back at the lab, Natalia's secretive past comes to light when Horatio asks for her help in the investigation.
Season 4, Episode 16
A registered sex offender living in Alexx's neighborhood dies after being attacked by four men near a park. The victim was allegedly accosting a girl just before his death. The investigation reveals that Alexx put up posters in the neighborhood warning parents that a sex offender was now living nearby. Horatio fears that the posters may have led to the murder and that Alexx could be held responsible in civil court.
Season 4, Episode 15
Walter Resden, an alleged serial killer who eluded Horatio 11 years ago in New York and again more recently in Miami, returns, and Horatio is horrified when he learns that the man is seeing the daughter of one of his alleged victims. The hunt for Resden begins when a man's body is found in a volleyball pit during a match and a house key is discovered in the victim's throat. Back at the lab, Natalia fears that she may be pregnant with Delko's baby.
Season 4, Episode 14
A dead man is discovered swinging from a noose attached to a drawbridge. Both of his eyes are shot out and a joker playing card is found in his pocket. Horatio believes the crime is a mob hit. The investigation reveals that the victim ran five of the hottest clubs in South Beach and that the gun used to kill him is connected to another murder believed to be mob-related. Also, Horatio takes Ryan off of the case after learning he was in a car accident caused by his poor eyesight.
Season 4, Episode 13
A man and a woman watching dancers at a beach pavilion are gunned down and Horatio believes the Mala Noche gang is responsible after he sees one of their tattoos on the dead man's chest. The investigation reveals that the shooter used a silencer. When a trace of an unknown drug is discovered on the woman's body, the probe shifts to the pharmaceutical company that manufactures it. Also, Delko learns that Horatio is seeing his sister, Marisol.
Season 4, Episode 12
A man attending a swinging party at a private downtown club is later found dead in one of the facility's bedroom suites with multiple ice-pick stab wounds in his neck and the letter “L” written on his chest. The victim was part of a seminar that taught men how to pick up women. Also, Delko's sister, Marisol, is arrested for possession of five bags of marijuana, a felony. Horatio tries to keep it quiet, but the information leaks out due to a mole in the department.
Season 4, Episode 11
Horatio searches for a rapist who attacked a woman six years ago in her new home after the man imprisoned for the assault is discovered to be innocent through a DNA test and released from jail. The wrongfully convicted man then brings a lawsuit against the police department. In the lab, Alexx discovers that a young woman who was thought to have been killed in a car accident actually died from a botched surgery.
Season 4, Episode 10
A drug lord is gunned down in his Coconut Grove mansion and a suspect arrested at the scene claims he sells marijuana to Delko. Stetler, from Internal Affairs, investigates and the case quickly gets personal when Horatio gets involved. As for the shooting, the CSIs learn that two gunmen may have been involved. Also, Calleigh makes a major decision concerning her job.
Three masked gunmen burst into and rob a Miami bank where Delko is one of the customers. A shootout claims the life of a teller and one of the thieves. The investigation soon reveals that the robbers are college students who are bringing to life a violent video game and that they are playing for points. Unfortunately, points are awarded for robbery, rape and murder. Later, Ryan receives some astonishing information about the crime lab that leaves him stunned.
Season 4, Episode 8
A woman, recently separated from her husband, is found dead in his house with four nails sticking out of her upper torso and neck. During the investigation, Ryan is shot in the corner of his right eye with a nail gun and rushed to the hospital. Back at the lab, Horatio is confronted by Stetler about the New York investigation into the death of the man who murdered Caine's mother more than 20 years ago.
A crossover episode that concludes on “CSI: NY” begins with a convicted serial killer who escapes after sabotaging an airplane that was flying him from New York to Miami, where he allegedly buried a body. After fleeing the crash site, the man goes on a killing spree and abducts a college student. Mac Taylor arrives to help Horatio track the killer since Mac originally arrested the man in New York. Darius: James Badge Dale. Johnson: Peter Parros. Alexa: Peyton List.
Horatio is suspected of killing his girlfriend after her body is found in a river. He insists he's being framed and that the real murderer is a serial killer he pursued 10 years ago in New York City. Meanwhile, Alexx becomes concerned about Delko when she learns that he's short on cash and fears that he's involved in something illegal.
Season 4, Episode 5
A 24-year-old pool boy working at a five-star Miami hotel is found dead on the hotel's lawn. The investigation reveals that he was very friendly with three wealthy housewives who were staying at the hotel and that he visited their penthouse suite shortly before he died. Horatio also learns that someone in the department is out to bring his team down.
A young man confesses to the murder of a woman on a boat and is sent to prison, but Horatio doesn't believe him and tries to prove that the man is innocent before the boyish-looking 23-year-old falls prey to fellow prisoners. The investigation into the killing reveals that the victim was going through a divorce and was selling the boat. But before Horatio can make a case, the confessed killer stabs an inmate to death in a cell.
Season 4, Episode 3
A high-school student from Georgia, who was in Miami for a class trip to the Bahamas, disappears after a wild night of partying at a downtown club. The next morning her rental car is found nearby with blood inside---but no body. Fingerprints are found on an outside window which suggests that someone was watching her, and a tiny video camera is found inside the car that shows the girl having sex in the backseat. Meanwhile, Stetler confronts Horatio about a case in Horatio's past in New York.
Two family members are killed after their yacht catches fire in the Atlantic and they jump overboard where sharks attack them.
A gang-related shooting occurs in a cemetery near a church where Horatio is confessing to a priest about a killing that took place years ago in New York and still haunts him. The gunfire interrupts his confession, and when he runs outside, he discovers a prisoner dead and a correctional officer dying. The investigation leads to Mala Noche, Miami's so-called new Mafia, whose leader wants Horatio dead.