A U.S. Navy nuclear engineer pleaded guilty on Monday to charges that he tried to sell some of America’s most closely guarded submarine secrets to a foreign country, in an agreement that will most likely send him to prison for 12 years or more.

The engineer, Jonathan Toebbe, was arrested last October with his wife, Diana Toebbe, and both had initially pleaded not guilty to one count of conspiracy to communicate restricted data and two counts of communication of restricted data. F.B.I. agents laid out a detailed account of how the Toebbes wrote to a foreign country offering to sell submarine nuclear reactor secrets in exchange for cryptocurrency.

Under the plea agreement, entered in federal court in Martinsburg, W.Va., Mr. Toebbe pleaded guilty to a single charge of conspiracy to communicate restricted data. Ms. Toebbe was not part of the plea agreement.

But as part of the plea agreement, Mr. Toebbe admitted that his wife was part of the conspiracy to sell the secrets and served as a lookout when he deposited information at the dead drops set up by the F.B.I. undercover agents.

In the agreement, he acknowledged that he sent the initial message to the foreign country, which has not been identified, and then communicated with the undercover F.B.I. officer. But, in the agreement, Mr. Toebbe said his wife “committed multiple overt acts in furtherance of the conspiracy.”

As part of the deal, Mr. Toebbe will help the government recover the cryptocurrency that undercover F.B.I. agents paid him, as well as the classified information he said he had but that he never turned over.

Mr. Toebbe had faced the possibility of life in prison, but under terms of the deal will instead face about 12 to 17 and a half years in prison.

The case received broad attention, raising questions of how a couple in a suburban home in Annapolis, Md., could become ensnared in an international spy plot.

A key question is whether Mr. Toebbe’s plea deal will result in any leniency for his wife, a former teacher at a private school in Annapolis. While Mr. Toebbe had done little to contest his pretrial detention or the charges against him, Ms. Toebbe’s lawyers had mounted a defense that she knew nothing of the plot to steal secrets. While she had gone to the dead drops, set up by undercover F.B.I. officers, she did not know of the scheme, her lawyers said.

The charges against Ms. Toebbe took her colleagues and former students by surprise, and many began wondering if the dissatisfaction she expressed about American politics in the classroom had taken a more sinister turn.

In her initial detention hearing, her lawyer said that while she did complain about former President Donald J. Trump, she did not betray her country.

But officials briefed on the investigation had portrayed Ms. Toebbe as being involved in the plot to sell the secrets her husband had meticulously stolen from the Navy over the course of many years.

Still, despite repeated attempts to have a court end her pretrial detention, the magistrate judge had refused, ordering her held without bail.