How can I start?I have to note that in all of the Spy School books so far, the plot can be divided in three main themes: the action, the humor and the relationships.I'll start from the action.This time the movement of the story was simply EPIC. The beginning seemed a little too "normal", and the return of the Pirncipal (intentional mistake) was kind of annoying but funny.However, there's a different feel to this book respect to the others; far from SPYDER, an actual sanctioned operation, Mike's return.All three of the themes have "elevated" in this fourth book.For once, the mission is a real mission, which gives it a sort of "professional feel" to it: no more escaping SPYDER, no more weird job offerings or such.The action starts right away, with investigation, infiltrations and very funny undercover conversations.*SPOILER* And when I mean the action elevated, it elevated: nuclear bombs, helicopters and machine guns.A lot more action scenes compared to the other books, I think- and with the combination of snow, skis and menacing bodyguards makes it an exciting and enjoyable read.The "villains's big scheme" is well thought of too; one of the hardest things of writing a spy book, to me, is to create a believable scheme. Not something too silly nor ridiculous (like world domination), but a plan acceptable and carefully crafted.Now let's move on to the humor.Not a single one of Stuart Gibbs's books lacks humour; in the worst situations or in a tense moment, there are always funny side remarks or comments (when Warren clocked his head and was like "I want a pony")Most of the humour, this time, was directed toward Erica- *SPOILER* from her personality flipping like a coin whenever she went undercover, or actual her mistakes, or even when Zoe tried to spark a conversation about "girl talk" and Erica was like what the heck are you talking about??It's kind of too bad though, as Erica's one of my favorite characters and seeing her act as the craziest teenager ever (even if just a cover) was both hilarious and unsettling.Then, finally, the relationships.I was betting a million dollars that Ben would get yet another crush on the enemy agent.And it did, as everyone predicted.Jessica is an interesting character- as always, not much of a description except that she has hair and is comparable to Erica, but that's even more vague because there wasn't much description of Erica either.She seems very...real. Not that the others seem fake; she's a well crafted character who will probably appear in the future.It's revealed that Zoe absolutely dislikes Warren, poor him, but I take that back because he was being a jerk the whole time.As for Mike, his presence definitely turned the tables; the part where he was calculating which girl was better was pretty funny.He's a smart kid, probably worthy of joining the Spy Academy, which will be interesting to see how Ben will react to that.And of course, I can't skip the big Stuart-Gibbs-trademark cliffhanging ending and the jump of Ben&Erica's relationship, can I?I had accidentally read a comment and spoiled the discovery, but that part was still unexpected and epic.To finish up, this was, in my opinion, the best book of the series so far.Things have really elevated, more characters have risen up in importance, more questions remain unanswered.But if anyone can continue the series and keep up the awesomeness, I'm sure Stuart Gibbs won't disappoint us.