Major Metcalf is first mentioned by Mrs. Boyle, who she's left in charge...
Agatha Christie's classic who-done-it, The Mousetrap, is celebrating its 67th year (eight months shy of the reign of Queen Elizabeth II) of being the longest-running play in the world, with over 27,500 performances to date.
Major Metcalf is first mentioned by Mrs. Boyle, who she's left in charge of the luggage she left outside Monkswell Manor in a blizzard. The first thing we learn about Major Metcalf is that he's extremely accommodating.
As he enters Monkswell Manor in act 1, scene 1, Major Metcalf is described by the playwright in the stage notes as "a middle-aged, square-shouldered man, very military in his manner and bearing."
On his entrance we see that he's amiable, polite, and respectful of women. He removes his hat for Mrs. Boyle, even though she left him in the blizzard with her luggage. He remarks that he hasn't seen this kind of weather "since I was on leave in nineteen-forty," which give us another hint about his military background.
A quick exchange between Giles and Major Metcalf seems to confirm his military history.
GILES: (authoritatively) Major!
MAJOR METCALF: (instinctively the soldier) Sir!
In scene 2, Major Metcalf is on the sofa in the main room reading a book, and Mrs. Boyle is sitting in an armchair writing a letter. Mrs. Boyle is complaining bitterly about everything, including the breakfast, the lunch, and the fact she wasn't given the best bedroom.
Major Metcalf does his best to look on the bright side of things, but Mrs. Boyle's pessimism eventually wears him down, and he takes the first opportunity to get away from her, even if the opportunity is to shovel snow away from the back door.
MAJOR METCALF: I'll give you a hand, what? Good exercise. Must have exercise.
When he returns to the others, he learns that the police have been summoned, shortly after Paravicini dropped the poker in the fireplace, startling Major Metcalf, who, according to the stage notes, "stands a though paralyzed." This might be a hint as to a lingering effect of Major Metcalf's wartime experience.
Later in the scene, a policeman, Sergeant Trotter, arrives. He says he's been sent to investigate "the Longridge farm case," and he questions everyone at the Manor about the case.
MAJOR METCALF: ... Read about the case in the papers at the time. I was stationed in Edinburgh then. No personal knowledge.
Without knowing much about the case, or so he says, Major Metcalf accuses Mrs. Boyle of being the magistrate who sent three children to a foster home at Longridge Farms, where they were abused and one child died. Mrs. Boyle admits that she was the magistrate, but she denies she did anything wrong and argues that she was simply doing her duty.
Mrs. Boyle is found dead, murdered, at the end of act 1.
In act 2, Major Metcalf tries to be helpful to Sergeant Trotter's investigation of Mrs. Boyle's murder, but his efforts seem to serve little purpose. In fact, suspicion for the murder falls on Major Metcalf for a time, but he holds up well under the pressure.
In due time, the murderer is revealed, as is Major Metcalf's true identity.
MAJOR METCALF: ...I've had my suspicions about him [the murderer] all along.
MOLLIE: You did? Didn't you believe he was a policeman?
MAJOR METCALF: I knew he wasn't a policeman. You see, Mrs. Ralston, I'm a policeman.
Major Metcalf explains his suspicions, how the Longridge Farm incident relates to Mrs Boyle's murderer, and how he discovered who the murderer was.