The most intense day of the whole trip: crossing into Spain and back over a dissolving border, riding through gorges carved by the Tagus, spotting vultures over sheer cliffs, and hugging a 3,350-year-old olive tree — the oldest in Portugal. Five bridges, one unforgettable day, ending in Abrantes.

What if a journey could be measured not in kilometers, but in bridges? This is the first of five chapters tracing the Tagus from Lisbon to the Spanish border and back — sixteen crossings in all. Chapter one takes us from the river's mouth through golden Alentejo plains to Marvão, a village perched like a stone nest between sky and earth.