When Ambassador Spock is captured on the Romulan world of Constanthus leading the Unificationist movement, Starfleet dispatches Enterprise with Admiral McCoy as an advisor to help negotiatiate his release. Mr Scott, enraged by Spock's incarceration, plans a rescue attempt on his own, by commandeering USS Yorktown, an antique vessel of the twenty third century.
The plot thickens when Scott too is captured by Romulans and McCoy jeopardizes the mission by his impulsive and reckless behaviour. Riker, Data and La Forge, on Picard's command, penetrate the Romulan space in a shuttle to rescue Scott. The Enterprise D team too is captured while trying to rescue Scott and as fate would have it, it is Scott, who by that time had escaped Romulan custody, who turns up to rescue them. The foursome then heads for Constanthus where Spock and his students, by that time, are getting ready to be hanged on charges of treason - though the political game between the Constanthus governor Tharrus and Proconsul Eragian in order to take credit for capturing the unificationists give them a chance of escape, in which they fail. They are beamed aboard the USS Yorktown just in time.
The Yorktown, crewed by Scott, Riker, Data, La Forge and Spock almost escaped but it was pursued by three Romulan warbirds and was on the verge of being destroyed when the USS Enterprise shows up and battles the Romulans and incapacitates them, only to detect three more incoming warbirds. McCoy finally manages to talk the Romulans into letting them go, as the Romulans can't afford to start a firefight with the Federation with the Romulan military stretched so thin in the region because of the Stugg problem. The Romulans ultimately escort the Enterprise and Yorktown to the Neutral zone and...freedom.
Spock may have been captured once, but this didn't deter him from his untiring effort to unify the Romulan star empire with the planet Vulcan and in the end of the novel, heads towards the Romulan empire, again. The chemistry between the old Enterprise crew and their acts are.....well, as if I need comment on that. They're...glorious!(Yes, thats' the Klingon in me speaking).
Overall, Crossover is a good mixture of the old and the new, of tradition and the modern, when crews of both the Enterprises unite for a common cause. I'd give "Crossover" 4 marks out of five.