The Alien Factor, the second and final Ultra Thriller, was published in 1993.

From the back cover: Tom Swift has caught a falling star - a visitor from outer space who is as beautiful as she is strange. But his secret encounter has set off alarms at the highest levels of government. A top-secret intelligence agency believes he's involved in an espionage and smuggling ring. To check Tom out, the Network sends two of its top operatives: Frank and Joe Hardy. But when the alien is kidnapped by a sinister industrialist, Frank and Joe and Tom realize they have to work together. They're dealing with a conspiracy that stretches from the farthest reaches of space into the deepest recesses of their own government. The fate of the country and the planet could rest on uncovering the shocking truth about the girl from another world!

The boys' time-travel exploits in the previous book have conveniently wiped away their memories of the time they spent together, making them near-strangers once again. This allows Frank and Joe to be initially suspicious and distrustful of the Swifts.

For the most part, the plot of this one was entirely forgettable.

Despite the creators' use of the Franklin W. Dixon pseudonym, the Ultra Thrillers were definitely more sci-fi than mystery, tying in with the Tom Swift paperbacks that Archway was publishing at the same time. Given the poor showing of those books, it's not surprising that the Ultra Thrillers only made it to two volumes.