Mark-Paul Gosselaar Mark-Paul Gosselaar

Mark-Paul Gosselaar will trade his Raising the Bar three-piece suit for skimpy swim trunks when he plays a tri-athlete from Dr. Maura Isles' past on Rizzoli & Isles.

"We had a couple scenes where he's swimming. Let me tell you, I have not enjoyed a swimming actor for a long time," star Sasha Alexander, who plays Isles, told

Exclusive: Mark-Paul Gosselaar to guest-star on Rizzoli & Isles

Gosselaar guests as Garret Fairfield, one of three brothers from a wealthy family in Boston. When Garret's brother is found dead, he and Isles reconnect.

Not only does his character's swimming background play a part in the episode, but Fairfield "shines a light on [Isles'] background and where she came from, [which causes] tension between her and Angie [Harmon]'s character," Gosselaar says.

TNT renews The Closer, Rizzoli & Isles and Leverage

Alexander offers a warning to Saved By the Bell fans: "He does some real shady stuff here. Zack Morris, how could you?!"

The episode airs Monday at 10/9c on TNT.

Additional reporting by Joyce Eng