To get into the International Spy Museum’s new fourth-floor vault, we bore a hole in the ceiling, lowered ourselves using a rope and put a guard to sleep with a drug-tipped dart after taking down the surveillance camera with just two keystrokes.

OK, they just let us in.

The museum, which opened in the Penn Quarter neighborhood of Washington in 2002 and moved to a bigger space in L’Enfant Plaza in 2019, has more than 10,000 objects in its collection, including statues, pens, disguises, listening devices and books used all over the world in the service of professional espionage.

As with most museums, a vast majority of those objects are not on display. And until a few weeks ago, they were far away, stored at a location outside the capital — making it a challenge for museum historians to reach the objects for study and preservation.

In 2020, the museum began consolidating its collection in its new building, a project that it completed this year.

Many of the artifacts in the vault came from one man: H. Keith Melton, a founding board member of the museum, who became one of the world’s renowned spy collectors. He is not a former intelligence agent himself; rather, he made his money as one of the country’s largest McDonald’s franchise owners. A condition of his donation, which he first pledged in 2016, was that the collection would eventually be moved to the museum itself, Mr. Melton said.