Enterprise encounters a time rift from which another starship emerges.
This is a classic episode that packs a lot of entertainment into a clever 45 minutes.
Like most time travel stories it has questionable plot elements but it is a highly enjoyable concept that hits you with something cool in the opening act and carries you through to the end.
Time travel adventures have been done to death over the years but when this was written it was a relatively fresh concept for television, albeit with a few recycled themes from the original Star Trek series. For me the concept of the past ship affecting the timeline is a great central plot and a past character returning within the altered timeline is a quite brilliant way of redoing something that was done exceptionally badly in a previous episode.
It's this guest character arc and a number of other moments for other characters that provide the best parts of the episode. Picard is put in a dilemma where he must make a difficult decision and has a number of excellent scenes in command. Guinan's role, although a bit contrived to make the plot function, does give her an air of wisdom and makes her an important member of the Enterprise crew. Geordie is in the thick of the action during the most intense scenes. Data has one quite subtlety good scene in the turbo-lift which I found quite memorable.
Visually it is one of the best Star Trek episodes with some excellent cinematography, editing, production design and sound effects. The look of the Enterprise is superb and this is further distinguished by the lighting. There is a very suspenseful action sequence towards the end which is one of the highlights of the franchise.
All performances are strong with Patrick Stewart excellent as ever. Others who stand out are Caryn Johnson (as Guinan), Denise Crosby, Tricia O'Neil, Christopher McDonald and LeVar Burton.
For me it's a 9.5/10 but I round upwards.
