Cleo, an eight-months-old hooded seal, was found in shallows on Spain's Costa del Sol, severely underweight.
An airline gave her a free lift from Malaga to London.
But a connecting flight to Scotland was cancelled and she ended up being taken by van to the National Seal Sanctuary at Gweek, in Cornwall.
There she has recovered to a healthy 50kg - nearly eight stone - making her ready for the next part of her long journey home.
The third leg is an overland trip of more than 800 miles along the length of mainland Britain to Wick, near John O'Groats - either by road or by rail.
From there, she is to be taken by boat to join a colony of seals in the Arctic.
Hooded seals are rarely seen as far south as Britain, let alone Spain.
One was found at Treyarnon Bay, Cornwall, in 1995, and another survived beaching at Little Haven, Pembrokeshire, in July this year.
The species is seen only very seldom on the Orkney and Shetland coasts.
It is not clear how Cleo came to wander so far from her normal feeding grounds in the North Atlantic.
Mark Steward, who has helped care for her, said: "Maybe there's a lack of fish up there and they are moving further south to try and find food."
Over-fishing by human rivals may be to blame.
Cleo should reach home just in time for the coldest weeks of the sunless Arctic winter.