Maddy, Sal and Liam, flush from their adventures in the first novel, almost have things back on track. Maddy and Liam travel back to San Francisco hours before the Great Earthquake to retrieve a batch of clone fetuses the agency has stored there to replace the ones lost in the power outage from the first story. Maddy finds a note addressed to her warning of an unnamed threat, but she's not to tell anyone else about it. Meanwhile in the year 2015, a group of Texas high schoolers takes a field trip to an energy development company. Among them is Edward Chan, who will eventually formulate the original time travel theories. A message arrives from the future, warning the TimeRiders that Chan is about to be assassinated on the trip. Liam is dispatched into the future with a freshly grown teen-aged but female version of their field unit, Bob, in tow.
The assassination attempt goes awry as an unforeseen event triggers the volatile energies into an explosion and strands most of the class, including Liam and Bob, in the prehistoric era. A new menace awaits them there, more dangerous than anything they could anticipate. And even as Maddy and Sal try to figure out how to rescue Liam, they face their biggest threat yet as the resulting timeshift has given rise to a new adversary... one who knows how and more importantly, when to find them.
The pacing and detailing in this second installment are better than the first; everything flows smoother and there are more layers to the story. But that's where the problems come in. The outline is already growing stale: time is altered-Liam & Bob go forth-Liam & Bob get lost-Maddy and Sal get in trouble, delaying their rescue attempt-Liam & Bob must come up with a way to let them know how to find them, etc. There's also still too much telling instead of showing- an important secret hinted at early on leaves no doubt it will be revealed before the end of the book, killing the story's momentum. We're only two books in and the heart of the entire nine-book story arc is all but laid out like a cookie crumb trail in the woods.
The villains are still slightly behind the heroes, meaning their challenge rating has been adjusted to the YA level. Illustrating this are several actions (not) taken by some of them that didn't make any sense at all. And one should always be careful plotting time travel stories- there's a couple of glaring plot holes which render the whole thing moot. Without going into detail I'll say that logic dictates that Time Travel should never have been discovered or at least it's been delayed a few more decades. And that the TimeRiders should've been shut down. But all of that presumes that humans even evolved in the first place.
There's more to like in this installment than the first one, but once again it gets bogged down by the details. And at this point it shouldn't be.