Actéon belonged to a fairly uniform series of thirty-one French patrol submarines that sailors nicknamed the "1,500-tonners" for their displacement. Engineers called them deep-cruising boats, and the figures explain the name. On the surface she stretched 92.3 metres and drew power from two diesels good for 17.5 knots; submerged, her electric motors moved her more modestly to ten. She could dive to eighty metres and run ten thousand nautical miles without refuelling. Laid down at the Ateliers et Chantiers de la Loire in Saint-Nazaire in July 1927, she slid into the water in April 1929 and was commissioned in December 1931. A submarine like this was designed to cross oceans, not to lurk in coastal shallows, which makes the manner of her death all the more striking.

When war came in 1939, Actéon hunted German ships. That December she joined her sister boat Fresnel in sweeping the central Atlantic for a German supply vessel, and by February 1940 she was working out of Casablanca, watching the sea lanes off the Canary Islands where Berlin was thought to be hiding cargo ships to refuel its U-boats. In April she shifted to the Mediterranean, based first at Bizerte in Tunisia and then at Beirut. There something unusual happened. She fell under the command of Admiral Andrew Cunningham, Britain's Commander-in-Chief in the Mediterranean, and patrolled the Dodecanese islands between Leros and Rhodes. A French crew, taking their orders from the Royal Navy, all of them hunting a common enemy.

Then France fell. German forces crossed the frontier in May 1940, Italy declared war in June, and by the twenty-fifth the armistices had taken effect. Actéon was recalled to Beirut, and overnight her war changed shape. She now served Vichy France, the rump government that ran the unoccupied south. Her batteries were failing and could not be replaced so far from home, so in October she limped back to Toulon under escort, and in 1941 she was sent on to Dakar in Senegal. When a British aircraft ordered her to stop along the way, she ignored it and pressed on, a small act that measured how far the compass had swung. The men aboard had not changed. Their country's place in the war had.

On 8 November 1942 the Allies came ashore in North Africa in Operation Torch, and the submarines that had once served alongside them were ordered out to oppose the landings. At five past two that morning Actéon received the command to put to sea. By evening she had found what she was looking for: the escort carriers Biter and Dasher off Cape Falcon, near Oran. She slipped through their outer screen, then surfaced to sharpen her firing angle, a gambler's move in waters thick with enemy warships. At nine o'clock the destroyer Westcott sighted her at nine hundred metres. Actéon dove, and minutes later Westcott's depth charges sank her where she lay, with no survivors. The boat that spent the first winter of the war fighting Germany died fighting the British, off a foreign coast, on the very day the tide of the whole war began to turn.