When Spock has almost completely recovered, Kirk decides that it is time for his crew to leave their exile on Vulcan. They take the captured Bird-of-Prey they named "H.M.S. Bounty" back to Earth when an alien probe threatens the planet by disabling all power systems. Spock finds out that the probe is on the search for traces of the now extinct humpback whales. In order to provide the probe with an answer, the crew travels to the late 20th century and find two such whales in the Cetacean Institute in Sausalito. After some problems with 20th century customs and technology, they manage to take the whales with them, together with biologist Gillian Taylor, who insists on staying aboard. Back in the 23rd century, all charges against Kirk except for noncompliance with his orders are dropped. Reduced to the rank of captain, he takes command of the new Enterprise NCC-1701-A.