Young Sherlock Holmes

Teenagers Sherlock Holmes (Nicholas Rowe) and John Watson (Alan Cox) meet and become good friends as students at London's less-than-prestigious Brompton Academy. Watson is introduced to Holmesâ mentor, Rupert T. Waxflatter (Nigel Stock), a retired schoolmaster and inventor. Waxflatter's niece Elizabeth Hardy (Sophie Ward) is also Holmesâ close friend and love interest. Holmes is generally recognized as brilliant but considered an undisciplined troublemaker by most of the school administration. He is closest to Professor Rathe (Anthony Higgins), his fencing instructor, who warns Holmes that he is too emotional and impulsive.
Meanwhile, a hooded figure uses a blowpipe to shoot Bentley Bobster and Reverend Duncan Nesbitt with hallucinogenic thorns, causing the men to experience nightmarish hallucinations resulting in their deaths. Holmes suspects foul play but is rebuffed by Scotland Yard policeman Lestrade (Roger Ashton-Griffiths) when he suggests a connection between the deaths. Holmes is unjustly expelled from the Academy due to a rival student Dudley's machinations, having had Holmes framed for cheating. As Holmes reluctantly prepares to leave, Professor Waxflatter is shot with a hallucinogenic thorn and accidentally kills himself while trying to fend off imaginary gremlins. As Waxflatter dies he whispers the word "Eh-tar" to Holmes.
Holmes secretly meets with Watson and Elizabeth and begins his investigation of the murders. Piecing together the clues â a jingling-bell sound made by the killer, a piece of cloth, and the blowpipe dropped at Waxflatter's murder scene â the trio uncover the existence of Rame Tep, an ancient Egyptian cult of Osiris worshippers. The cultâs main weapons were blowpipes, used to shoot thorns dipped into a solution made of plant extract which causes the victim to experience realistic, nightmare-like hallucinations. They track the cult to a London warehouse where the Rame Tep are conducting human sacrifices in a secret underground wooden pyramid. After interrupting one of their ceremonies, Holmes, Elizabeth, and Watson are chased by the Rame Tep and shot with thorns, barely escaping with their lives into a cemetery. They all experience hallucinations (Elizabeth that she is being chased by the undead, Watson that his favorite stuffed pastries are coming to life and force feeding themselves to him, and Holmes that his dead father is furious with him), but Holmes is able to keep them all level-headed and they survive.
The following evening, at Waxflatterâs loft, Holmes and Watson discover a picture of the three victims and a fourth man, Chester Cragwitch (Freddie Jones). However the three are discovered by Professor Rathe and the school nurse Mrs. Dribb (Susan Fleetwood) who prepare to expel Watson and Elizabeth in the morning. That night, while Elizabeth heads to Waxflatter's loft to salvage his work, Holmes and Watson locate Mr. Cragwitch who explains that in his youth he and the other men had discovered an underground pyramid of Rame Tep while planning to build a hotel in Egypt. Their find led to an angry uprising by the local populace which was violently put down by the British Army. A local boy named Eh-tar and his sister vowed revenge after their parents were killed. Cragwitch is then shot by a poisoned thorn and tries to kill Holmes, but he is knocked unconscious by Inspector Lestrade who reconsidered Holmes' advice after he himself was accidentally poisoned by the hallucinogen.
As they return to the school grounds, a chance remark by Watson causes Holmes to realize that Rathe is Eh-Tar, but he and Watson arrive too late to stop Rathe and Mrs. Dribb, who is revealed as Eh-Tar's sister, from abducting Elizabeth. Using Waxflatterâs self-propelled, heavier-than-air flying machine, they travel to the warehouse just in time to prevent Rathe from sacrificing Elizabeth, setting the cult's pyramid temple on fire. As Rathe escapes with Elizabeth, Mrs. Dribb swallows one of her own hallucinogenic thorns in a fight with Holmes. Watson successfully thwarts Ratheâs escape by sabotaging his carriage. Rathe then tries to shoot Holmes, but Elizabeth intervenes and is wounded instead. Enraged, Holmes duels Rathe and manages to get the better of him when Rathe falls through the frozen River Thames. Holmes returns to Elizabethâs side and holds her as she dies.
Afterwards, as he exchanges goodbyes with Watson, Holmes explains how he figured out the identity of Rathe and Dribb, with Watson pointing out that "Rathe" is "Eh-Tar" spelled backwards â a clue that Holmes had failed to notice. Holmes says he will transfer to another school to take his mind off Elizabeth. Watson gives his friend a pipe as a farewell present. As Holmes leaves by horse carriage with his new detective outfit, Watson's older self (the Narrator) expresses that he was certain he would have more adventures at Holmes' side.
In a post-credits scene, Rathe is revealed to be alive; he checks himself into an Alpine inn with a new name, âMoriartyâ, foreshadowing his role as Holmes' future nemesis.

Question: What does Holmes explain to Watson after exchanging goodbyes ?
Answer: How he figured out the identity of Rathe and Dribb.