Not even Captain Kirk could have saved Star Trek: Enterprise.The UPN series (Fridays, 8 pm/ET) will end its four-year run May 13 due to a steep drop in viewers (just 2.9 million this season). "And I don't think an appearance by William Shatner would have mattered," says executive producer Rick Berman. He'd been in talks with the Trek icon, hoping that a guest shot would lure lapsed Trekkers. "But the writing was on the wall last year," he explains, "when UPN moved us to Friday nights and the network began skewing in a different direction." (That would be toward young female viewers.) On the bright side, Berman promises that Enterprise will end with "a little valentine to all Trek fans. I will say that the finale is surprising — something we've never done before — and it might involve people from another Star Trek show." Forget that teasing "might." The space-opera honcho will make it so: Star Trek: The Next Generation vets Jonathan Frakes (William Riker) and Marina Sirtis (Deanna Troi) have just signed on to guest-star in Enterprise's last hurrah.