Seven of Nine discovers indications of a Starfleet vessel via an ancient communication network that extends to the Alpha Quadrant. Attempts at subspace communication fail, and with that vessel moving out of range and time running out, they decide to send the EMH, who successfully arrives on the U.S.S. Prometheus, a highly advanced Starfleet prototype with incredibly beautiful interiors, only to find the crew dead and the ship taken over by Romulans, the start of a great Doctor episode, one of the best of the series, with spectacular eye candy, lots of humor, plenty of opportunity for Robert Picardo to do his Doctor shtick (especially with Andy Dick playing the new EMH-2 on the Prometheus, giving the Doctor plenty of cause to be grumpy), and lots of action and gratifying scenes with ships of Nebula, Akira, and Defiant class, as well as the Prometheus and Romulan Warbirds.
It was also our introduction to the Hirogen, this time only Idrin (later seen in VOY: Hunters), played by seven-foot-tall Tiny Ron, who played Maihar'du, Grand Negus Zek's Hupyrian servent, in seven episodes of DS9.
But to nitpick a little, when the EHM dematerialized, so did his mobile emitter (which should have simply fallen to the floor), Captain Janeway's hairstyle was awful, and Andy Probert (who designed the Romulan D'Deridex-class Warbird) complained about the firing of a disruptor beam from the main deflector in this episode. The impulse exhausts were missing from the Warbirds. (When I was making an MSD of that vessel, he described where they should be after I had drawn them differently, explaining that they had been in a hurry and grabbed the drawings from this desk before he was finished. But Greg Jein, who made the model, built it without that, and Andy didn't check up on the work at that time. Therefore, they are always missing onscreen.) And when the Doctor returned, I would have expected him to eventually at least tell the captain about the new Starfleet uniforms. Changing to the new style might have been something of a morale booster for the crew.
Message in a bottle? Yeah, good one. Cool ship, that Promy.