Past Santarém, the Tagus opens up into rice-colored plains — the Lezíria do Tejo — and still holds four more crossings before delivering us back to Lisbon: a bridge that has never known lighting, a farm-to-table lunch in Cartaxo, a crossing signed by the same hand behind the Eiffel Tower, and a final stretch wrapped in carpets of green and yellow.

With eleven Tagus crossings already behind us, the road back begins. This chapter runs from Abrantes to Santarém: Almourol Castle, perched on its rocky island and the ultimate symbol of the Templars in Portugal; Golegã, the country's "horse capital"; the Chamusca Iron Bridge, a textbook example of early-20th-century ironwork; and the D. Luís I Bridge in Santarém, once the largest bridge on the Iberian Peninsula.

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.